1. Field of the Invention
The present invention relates to electrical generating systems for motor vehicles.
2. Description of the Related Art
Some alternators for motor vehicles have two voltage regulation modes. In the first mode ("normal mode"), the alternator's voltage regulator is coupled to a voltage sensing point near the vehicle battery. The voltage regulator uses this connection as feedback in its effort to regulate the alternator output such that the battery voltage is maintained at a target value. The second mode, which can be referred to as "fault mode," is entered if the voltage sensing connection near the battery becomes corrupted, for example by the sense wire becoming open- or short-circuited. In fault mode, a loop-back connection within the alternator from the alternator's DC power output to the voltage regulator is used by the voltage regulator as a surrogate for the battery voltage feedback which has been lost.
However, there is a concern in such a system with a "normal mode" and a "fault mode." If the fault mode voltage target (as sensed at the sensing point near the battery) is set too close to the normal mode voltage target (as sensed at the alternator's DC power output), the voltage regulator of the alternator may "fight" between controlling to the normal mode voltage target and the fault mode voltage target. Typically, it is desirable to have the two targets within about one volt or less of one another, the typical voltage difference between the output of the alternator and the vehicle battery. However, with the two targets this close, the aforementioned fighting within the voltage regulator may occur. Thus, a system which can provide close voltage targets without the concern for fighting within the regulator can provide benefits over alternative systems.
The typical electrical system of a motor vehicle is designed to operate at a nominal voltage of about 12 volts DC. Accordingly, the typical electrical generating system in a motor vehicle generates electricity at a nominal voltage of about 12 volts DC. However, some specialized electrical loads may desire voltages above 12 volts for certain limited periods of time. An electrically-heated catalyst in a catalytic converter is one such load. Thus, an electrical generating system which can provide temporary excursions to a higher voltage can be advantageous.
In any motor vehicle electrical system, providing effective and reliable functionality in a system with minimal complexity is of foremost interest. Thus, an electrical generating system which can maintain the fault-mode performance described above, which can provide for temporary excursions if desired to a higher voltage output and which performs all of these functions with minimal complexity will prove beneficial.